Method and apparatus for driving a CCD image sensor

ABSTRACT

A method and apparatus for driving a CCD image sensor is provided in which the time period of subscan can be a continuous value responsive to the amount of image information received by the CCD image sensor. The control time periods of the CCD image sensor are separated into first control time periods which are fixed in duration and second control time periods which are variable. The first and second control time periods alternate. Image information signals are obtained by exposure of the CCD image sensor during the first control time periods and are outputted during the second control time periods. These second control time periods vary in accordance with the amount of image information received by the CCD image sensor during the first control time periods.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for driving CCD imagesensor in facsimile type equipment in which the signal sensed by the CCDimage sensor is coded by using a redundancy suppressed coding technique.

In facsimile equipment using the above mentioned coding techniquebecause the amount of coded information varies for each main scan,intermittent and variable speed subscanning systems have been adopted.In these subscanning systems, the subscan stops when the informationstored exceeds a certain amount. The subscan then resumes when theinformation stored is less than a certain amount. However, in thesesubscanning systems, high speed operation is difficult because of thetime it takes to completely stop subscanning. In addition, during thestopping or starting of the subscan, disorders sometimes occur in thereproduced picture due to vibration of the mechanical system.

Other variable speed subscanning systems have been adopted to resolvethe above mentioned problems. In such systems, a predetermined number Nof subscan speeds are available for selection. By selecting the propercombination of subscan speeds from the predetermined number N of subscanspeeds, each main scan can be made to correspond to the amount of codedinformation.

Although the disadvantages of stopping and starting the subscan areavoided in the latter systems, other disadvantages occur as explainedmore fully below.

The exposure time period of a CCD image sensor is fixed at T seconds asshown in FIG. 1. When the first shift pulse P₁ (in conventional CCDimage sensors, shift pulses supplied to the CCD image sensor causeinformation stored in the image sensor to be shifted out so that newinformation can be stored in response to the next exposure) is suppliedto the CCD image sensor, exposure a₁ for particular main scan begins.When shift pulse P₂ occurs, exposure a₁ is finished and exposure a₂ forthe next main scan begins. Upon the occurrence of shift pulses P₃, P₄,P₅, . . . , similar exposures a₃, a₄, a₅, . . . , take place.Information stored in the CCD image sensor during exposure a₁ isoutputted in response to shift pulse P₂. The transfer output time of theinformation stored during exposure a₁ is represented by signal b₁.Similarly, information stored during exposure a₂ is outputted during thetime represented by signal b₂ in response to shift pulse P₃. At thetimes of shift pulses P₄, P₅, P₆, . . . , information stored duringexposures a₄, a₅, a₆, . . . , is outputted during transfer output timesb₄, b₅, b₆, . . .

As mentioned previously, in the above conventional system, the exposuretime period is fixed at T seconds. As a result, the outputting of storedinformation signals can only occur in the interval of an integermultiple of T. This means that, when the information stored in the CCDimage sensor cannot be outputted in one time period T, therebynecessitating further delay in the subscan time period, the subscan timeperiod must be an integer times T, such as one T, 2T, 3T, etc., which isnot a continuous value. When applied to a CCD image sensor in facsimileequipment, time is lost in outputting stored information from the CCDimage sensor because of the fixed time period T in the subscan. As aresult of this lack of control over the outputting of information duringsubscanning, the overall speed of the facsimile equipment is lessened.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method and apparatus fordriving a CCD image sensor in which the time period of subscan can be acontinuous value. In this regard, it is an object of this invention tomore precisely control the operation of a CCD image sensor so that thesubscan time period can be reduced and the speed of facsimile equipmentusing this method and apparatus can be increased.

In this invention, control time periods for the CCD image sensor areseparated into first control time periods which are fixed and secondcontrol time periods which are variable. The first and second timeperiods appear alternately. Information signals are obtained by exposureof the CCD image sensor during the first control time periods. Theseinformation signals are transferred or outputted from the CCD imagesensor during the second control time periods. The length of the secondcontrol time periods varies in response to the amount of informationstored in the line memory device so that the length of the subscan timeperiod, which is formed by the first and second control time periods,can be optimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a timing chart for a conventional method of controlling theoperation of a CCD image sensor.

FIG. 2 shows a timing chart for the method of controlling the operationof a CCD image sensor according to the present invention.

FIG. 3 is a block diagram of the main parts of facsimile equipmentaccording to a first embodiment of the present invention.

FIG. 4 is a diagram showing the relationship between the initial valueof the program counter of FIG. 3 and the ROM address of the maincontroller.

FIG. 5 is a block diagram of a modification of the detecting circuitshown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 shows a timing chart for explaining the method of driving a CCDsensor according to the present invention. In FIG. 2 T₁, T₃, . . .T_(2n-1) are first control time periods and T₂, T₄, . . . T_(2n) aresecond control time periods. First control time periods T₁, T₃, . . .T_(2n-1) have time periods which are determined by the characteristicsof the CCD image sensor. Upon exposure of the CCD image sensor, chargesare stored by the CCD image sensor during these time periods withoutsaturation of the CCD image sensor. Second control time periods T₂, T₄,. . . T_(2n) are variable length time periods which are longer than thetime it takes to output the charges stored in the CCD image sensor. Asshown in FIG. 2, first control time periods T₁, T₃, . . . T_(2n-1) andsecond control time periods T₂, T₄, . . . T_(2n) appear alternately.

As shown in FIG. 2(c), a series of shift pulses SP are supplied to theCCD image sensor after each of the time periods T₁, T₂, . . . T_(2n).Upon occurrence of the first shift pulse SP₁, time period T₁ begins.Upon expiration of time period T₁, shift pulse SP₂ is generated andsupplied to the CCD image sensor, which also initiates time period T₂.Upon expiration of time period T₂, shift pulse SP₃ is generated, etc.Upon expiration of every fixed time period T_(2i-1) and alternatevariable time period T_(2i), shift pulses SP_(i) are supplied to the CCDimage sensor.

Accordingly, signals b'₁, b'₃, . . . b'_(2n-1), which correspond tofirst control time periods T₁, T₃, . . . T_(2n-1), are initiated byshift pulses SP₂, SP₄, . . . SP_(2n), respectively. Signals b'₂, b'₄, .. . b'_(2n), which correspond to second control time periods T₂, T₄, . .. T_(2n), are initiated by shift pulses SP₃, SP₅, . . . SP_(2n+1),respectively. However, signals b'₂, b'₄, . . . b'_(2n), which are shownin dotted lines, are not used by the facsimile equipment of the presentinvention.

Due to the existence of variable second control time periods T₂, T₄, . .. T_(2n) among first control time periods T₁, T₃, . . . T_(2n-1), it ispossible to reduce the subscan time period formed by the first andsecond control time periods by making the subscan time period correspondto (T₁ +T₀), where T₀ equals the transfer output time period required tooutput the information stored in the CCD sensor during the previousexposure or first control time period. Thus, the maximum subscan speedis a function of the sum of the first control time period and time T₀.

An embodiment of facsimile equipment using the above method is shown inFIG. 3. An output signal of CCD image sensor 1 is supplied via analogswitch 2 to A/D converter 3 for conversion to digital form. A digitaloutput signal of converter 3 is temporarily stored in line memory device4 and then supplied to encoder 5 to be encoded and compressed. Anencoded signal from encoder 5 is modulated by modem 6 and thentrasmitted to its ultimate distination.

The current storage condition of line memory device 4 is constantlydetected by detecting circuit 7. Detecting circuit 7 counts down thenumber of line signals supplied from A/D converter 3 to line memorydevice 4 in counter 71 and the number of line signals read out from linememory device 4 by encoder 5 in counter 72. The difference between thesecounts is determined by up/down counter 73 and supplied to maincontroller 8.

Main controller 8 comprises a micropressor and memory which stores theprogram and data used by the microprocessor; the micropressor controlsthe operation of the whole facsimile equipment. In particular, maincontroller 8 receives difference signals from detecting circuit 7 whichindicate the storing state of line memory device 4 and then thiscontroller generates control signals which control the time periodsbetween the pulses generated by program counter 9 to thereby preventline memory device 4 from being overflowed or underflowed. That is, whenthere is a possibilty that line memory device 4 will overflow,controller 8 generates a control signal which lengthens the timeperiods. When there is a possibility that line memory device 4 willunderflow, controller 8 generates a control signal which shortens thetime periods.

Program counter 9 supplies scanning step pulses d shown in FIG. 2(d) tocounter 10 and a pulse motor (not shown) at desired times based on thecontrol signals from main controller 8. Counter 10 counts scanning steppulses d in response to reference clock signals by oscillator 11. Atfirst, when scanning step pulse d₁ is supplied from counter 9 to counter10, counter 10 puts out pulse SP₁ (FIG. 2(c)) through line 12 to CCDimage sensor 1 so that the signals stored in the photosensor cells ofthe CCD image sensor are transferred to a shift register and thenexposure of the photosensor cells begins. At the same time, counter 10puts out a control signal over line 14 to turn off switch 2.

Counter 10 also generates clock signals by dividing the frequency of thereference clock signals from oscillator 11. These clock signals aresupplied to image sensor 1 over line 13 during the time T₀ (FIG. 2(b))so that the signals stored in the shift register of CCD image sensor 1are outputted to switch 2. The number of clock signals generated bycounter 10 corresponds to the number of elements in the shift register.However, since switch 2 is turned off at this stage, the signalsoutputted from CCD image sensor 1 are not supplied to A/D converter 3.But when counter 10 counts up the number of reference pulsescorresponding to predetermined time period T₁ (FIG. 2(a)) from the timeof pulse SP₁, counter 10 generates pulse SP₂, which is supplied to imagesensor 1 over line 12 so that the signals stored in the photosensorcells are transferred to the shift register and exposure of thephotosensor cells begins. At the same time, counter 10 outputs a controlsignal over line 14 to turn on switch 2. Counter 10 further outputs theclock signals generated by dividing the frequency of the reference clocksignals over line 13 to image sensor 1 during time T₀ so that thesignals stored in the shift register are supplied to A/D converter 3through switch 2. The number of clock signals corresponds to the numberof elements in the shift register.

After outputting the clock signals, counter 10 does not generate anyadditional signals until the next scanning step pulse d₂. When pulse d₂is supplied from counter 9, counter 10 outputs pulse SP₃ over line 12and generates a control signal to turn off switch 2. Since these and theother steps described above are repeated, only the output picturesignals b'₁, b'₃, . . . b'_(2n-1) (FIG. 2(b)) out of the output picturesignals b'₁, b'₂, b'₃, . . . b'_(2n) are passed through switch 2. As aresult, the subscanning time periods T₁ +T₂, T₃ +T₄, . . . T_(2n-1)+T_(2n) vary. Since T₂, T₄, . . . T_(2n) have values larger than orequal to T₀, the subscanning time periods have values larger than orequal to T₁ +T₀. As a result, precise and optimum control of the amountof data temporarily stored in line memory device 4 can be achieved.

Next, the control of the scanning step pulses d in response to thestorage condition of line memory device 4 will be explained in moredetail. The digital signal from converter 3 is written in line memories41-44 via bus driver 4b under the control of controller 4c. Each timeencoder 5 outputs read request pulse r, data is read out from linememories 41-44 and supplied via latch 4l to encoder 5 under the controlof controller 4c. The addresses for writing data into line memories41-44 are provided by counter 45 by counting clock signal CK, which issynchronized with the clock signal on line 13 which outputs the signalsstored in the shift register of CCD image sensor 1. The addresses forreading out data from line memories 41-44 are provided by counter 46 bycounting the read request pulse r. The upper two bits of counter 45select the line memory at the time of writing and the upper two bits ofcounter 46 select the line memory at the time of reading out. Other bitsof counters 45 and 46 designate the memory address. When the counter incounters 45 and 46 corresponds to the number of elements of CCD imagesensor 1, the upper two bits are varied, e.g., from "00" to "01", from"01" to "10", from "10" to "11", or from "11" to "00".

Selectors 47 and 48 are switched by controller 4c so that the outputsignals from counters 45 and 46 are passed through at the time ofwriting and reading out, respectively. Controller 4c controls bus driver4b, latch 4l and selectors 47-48 through lines l1, l2 and l3 so thatinformation is either read out or written in line memories 41-44 inresponse to clock signals CK from counter 10 or read request pulses r.Decoder 49 decodes the upper two bits of counter 45 or counter 46, andselects one of the line memories 41-44.

Line counter 71 generates one pulse each time the number of clocksignals CK corresponds to the picture signal for one scanning line. Onthe other hand, line counter 72 generates one pulse each time the numberof read request signals corresponds to the picture signal for onescanning line. The output signals of line counter 71 are supplied to theup terminal of up-down counter 73 and the output signals of line counter72 are supplied to the down terminal. When picture signals for onescanning line are written in line memories 41-44, up-down counter 73counts up and when picture signals for one scanning line are read out,up-down counter 73 counts down. Accordingly, the count of counter 73indicates the number of line memories in use. This count also designatesaddresses in the ROM of main controller 8 where the initial values ofcounter 9 corresponding to the number of line memories which are beingused are stored. The relation between the initial values of counter 9and these addresses is determined in advance as shown in FIG. 4, forexample. Counter 9 down counts from the intial values in response to aclock signal and generates scanning step pulses d when the value becomes0. Here the time of counting from 10 (initial value) to 0 is equal tothe time (T₁ +T₀). Thus, the time periods between pulses d can be variedaccording to the number of line memories which are being.

FIG. 5 shows a modification of detecting circuit 7 of FIG. 3. In thismodified circuit, comparator 74 and another up-down counter 75 areconnected to the output of up-down counter 73. Comparator 74 comparesthe output value of up-down counter 73 and the output value of up-downcounter 75, and if the output value of counter 73 is larger than theoutput value of counter 75, comparator 74 actuates the up terminal ofcounter 75 so that counter 75 counts up by 1. If the output value ofcounter 73 is smaller than the output value of counter 75, comparator 74actuates the down terminal of counter 75 so that counter counts downby 1. Comparator 74 performs the above comparison each time counter 9outputs pulse d.

In the modified detecting circuit of FIG. 5, the output value fromdetecting circuit 7 to controller 8 does not change radically, i.e., thevalue designating an address of ROM cannot be changed by more than 1 foreach pulse d. As a result, the initial value of counter 9 does notchange radically and consequently the speed of the pulse motor is notsubjected to rapid variations.

Although illustrative embodiments of the invention have been describedin detail with reference to the accompanying drawings, it is to beunderstood that the invention is not limited to those preciseembodiments and that various changes and modifications may be effectedtherein by one skilled in the art without departing from the scope orspirit of the invention.

I claim:
 1. In facsimile type equipment having a CCD image sensor whichis exposed to receive and temporarily store image information signalsduring a first control time period of fixed duration and then outputthese image information signals during a second control time period, thefirst and second control time periods alternating with each other asimages are successively exposed to said CCD image sensor, a CCD imagesensor driving apparatus comprising:memory means coupled to said CCDimage sensor for receiving and temporarily storing only the imageinformation signals generated by said CCD image sensor during successivefirst control time periods, said memory means disregarding any imageinformation signals generated by said CCD image sensor during successivesecond control time periods; encoder means coupled to said memory meansfor successively encoding the image information signals read out fromsaid memory means; detecting circuit means responsive to the storagecondition of said memory means for generating a storage signalrepresentative of the current amount of storage of image informationsignals in said memory means; and control circuit means coupled to saiddetecting circuit means and responsive to said storage signal forgenerating control signals which are coupled to said CCD image sensor tovary the length of each second control time period in accordance withthe amount of storage of image information signals in said memory means,the second control time period being greater than or equal to the amountof time required to transfer the image information signals from saidmemory means to thereby optimize the time period of the second controltime period and increase the speed of the facsimile type equipment.
 2. ACCD image sensor driving apparatus according to claim 1 furthercomprising counter means coupled to said control circuit means forreceiving the control signals and generating scanning step pulses whichdetermine the length of the second control time periods.
 3. A CCD imagesensor driving apparatus according to claim 2 wherein said detectingcircuit means includes an up-down counter which counts up or down inaccordance with the image information signals incoming to said memorymeans.
 4. A CCD image sensor driving apparatus according to claim 2wherein said detecting circuit means includes a first up-down counterand a second up-down counter, said first counter counting up or down inaccordance with the image information signals incoming to said memorymeans and said second counter counting up or down by one according tothe result of comparing the output of said first counter and the outputof said second counter to thereby prevent any rapid variation in thestorage signal outputted by said detecting circuit means.
 5. A methodfor driving a CCD image sensor in facsimile type equipment comprisingthe following steps:generating first control time periods for said CCDimage sensor, the first control time periods having a fixed durationwhich said CCD image sensor is exposed to receive and temporarily storeimage information signals; generating second control time periods forsaid CCD image sensor, each second control time period hving a variableduration during which said CCD image sensor outputs the imageinformation signals temporarily stored in said CCD image sensor during atransfer output time period, the second control time periods alternatingwith the first control time periods and each second control time periodhaving a variable duration greater than or equal to the transfer outputtime period and which varies in accordance with the amount of imageinformation temporarily stored in said CCD image sensor during the firstcontrol time periods.
 6. A method for driving a CCD image sensoraccording to claim 5 further comprising the steps of:temporarily storingthe image information signals outputted by said CCD image sensor in amemory device; generating storage signals representative of the amountof image information currently stored in said memory device; and varyingthe duration of the second control time periods of said CCD image sensorin response to the storage signals.